The invention relates to a continuous process for the esterification of an alpha, beta-unsaturated carboxylic acid and a mono-alcohol.

A process of the above-mentioned type is described in an article by P. Lutze et al. entitled "Esterification of Acrylic Acid and n-Butanol in a Pilot-Scale Reactive Distillation Column, Experimental Investigation, Model Validation, and Process Analysis" in Ind. Eng. Chem. Res. 2012, 51 , Pages 16444-16456. This article deals with reactive distillation for the production of n-butyl acrylate. Experimental design and parameters such as distillate-to-feed mass ratio, reflux ratio, molar feed ratio, and top pressure of the column are presented. The chemical system is the heterogeneously catalyzed esterification of acrylic acid and n-butanol.

US 4280009 describes the continuous production of 2-ethyl-hexyl acrylate. The process is carried out in an installation having a reaction zone wherein acrylic acid, 2-ethyl-hexanol and sulfuric acid catalyst are continuously introduced. To the reaction zone a separate first distillation zone is mounted, wherein reaction water is azeotropically distilled off together with 2-ethyl-hexanol.

The process described in this document requires an installation having several units, and the residence time in the reaction zone is up to 8 hours. This limits the throughput and efficiency of the process, and further gives rise to a waste stream of high boiling residue.

Polymerizable esters of mono alcohols and alpha, beta-unsaturated carboxylic acids are useful raw materials for the preparation of polymers, in particular acrylic and methacrylic copolymers. Such polymers find use in many industries, for example as film-forming binders in coating compositions. Esters of monoalcohols having 5 or more carbon atoms are of particular interest, because such esters confer specific desirable properties to the polymers and co-polymers. Furthermore, it is desirable to use certain alcohols having 5 or more carbon atoms and which are available from renewable resources in industrial processes. However, the known processes have drawbacks or are not suitable for the esterification of an alpha, beta-unsaturated carboxylic acid and a mono-alcohol having 5 or more carbon atoms. Accordingly, there is a need for improved processes for the preparation of esters of alpha, beta-unsaturated carboxylic acid and a mono-alcohol having 5 or more carbon atoms, and wherein the process is carried out in a simple installation. The process should allow for a high throughput, leading to short residence times of the polymerizable monomers to avoid undesirable waste of high boiling residue. High throughput and short residence times give rise to additional benefical effects, in particular reduced capital expense, reduced energy consumption, and reduced operating costs.

The present invention provides a continuous process for the esterification of an alpha, beta-unsaturated carboxylic acid and a mono-alcohol having 5 or more carbon atoms in a reactive distillation chamber, comprising the steps of

a) feeding a first feed stream comprising the alpha, beta-unsaturated carboxylic acid to the lower half of the reactive distillation chamber and feeding a second feed stream comprising the mono-alcohol having 5 or more carbon atoms to the upper half of the reactive distillation chamber so as to cause a countercurrent flow of alcohol and carboxylic acid in the reactive distillation chamber, wherein the feed molar ratio of mono- alcohol having 5 or more carbon atoms to acid groups of the alpha, beta- unsaturated carboxylic acid is in the range of 0.95 to 1.20,

b) causing the alpha, beta-unsaturated carboxylic acid and the alcohol to form an ester in the reactive distillation chamber by heating, wherein the temperature in the bottom part of the reactive distillation chamber is in the range of 130 to 250 °C, and wherein the pressure in the reactive distillation chamber is below atmospheric pressure,

c) removing reaction water by distillation, and optionally separating organic material from the distillate, and recycling any separated organic material to the reactive distillation chamber, and

d) removing the formed ester from the bottom of the reactive distillation chamber.

The process allows for the preparation of esters of alpha, beta-unsaturated carboxylic acids and monoalcohols having 5 or more carbon atoms in an efficient way, and with esters of high purity being obtained. The process is carried out in a simple installation and allows for a high throughput, leading to short residence times of the polymerizable monomers. Undesirable waste of high boiling residue is largely avoided.

Typically, the formed ester is removed at the bottom of the reactive distillation chamber in a purity of 95 % by weight or higher, preferably 97 % by weight or higher.

It is to be understood that, although the process steps are described sequentially, the different steps of the continuous process take place simultaneously. Esterification and distillative separation occur in a single integrated reactive distillation chamber.

The process is carried out in a reactive distillation unit having a reactive distillation chamber having a lower half and an upper half, a feed line for feeding alpha, beta- unsaturated carboxylic acid to the lower half of the reactive distillation chamber, and a feed line for feeding mono-alcohol having 5 or more carbon atoms to the upper half of the reactive distillation chamber. At the bottom of the reactive distillation chamber is a line for removing the formed ester from the unit. At the top of the reactive distillation chamber is a line for removing vapour components from the reactive distillation chamber to a condenser connected to a condensate receiver to receive the distillate. In one embodiment, substantially all vapour components are condensed in the condenser. However, it can also be sufficient to partly condensate the vapour. Water, for example, may also be removed from the process partly or entirely as water vapour. The condensate receiver or an additional unit suitably has means to separate water and organic material, for example by phase separation. In all cases the reactive distillation column is operated with an external reflux, i.e. at least a portion of the condensed overhead liquid product from the reactive distillation column is returned to the upper part of the column. Inside the column, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficiency of the distillation column. The reactive distillation unit furthermore has the usual means to control temperature and pressure. Examples of suitable alpha, beta-unsaturated carboxylic acids for esterification in the process of the invention include acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, and 2-butenoic acid. Acrylic and methacrylic are preferred acids for use in the process. Mono-alcohols have only one hydroxyl group. Other functional groups may be present in the mono-alcohols, provided that they do not react with carboxylic acid groups or with alpha, beta-unsaturated groups under the esterification conditions. Examples of suitable mono-alcohols having 5 or more carbon atoms include pentanol, hexanol, heptanol, octanol, cyclohexanol, and isomers and mixtures thereof. Preferred alcohols are 2-ethylhexyl alcohol, cyclohexanol, isoamyl alcohol, and abietyl alcohol.

As mentioned above, the temperature in the bottom part of the reactive distillation chamber is in the range of 130 to 250 °C, preferably the temperature is at least 140 °C, or at least 145°C, preferably in the range of 140 or 145 to 190 °C. It is to be understood that the bottom part of the reactive distillation chamber means the lower third of the reactive distillation chamber.

The temperature range in the upper third of the reactive distillation chamber generally is in the range of 45 to 100 °C, preferably 50 to 80 °C.

The temperature is selected to obtain a sufficient reaction rate. The maximum temperature in the bottom may be limited by the stability of the alpha, beta- unsaturated carboxylic acid or ester or their tendency to polymerize.

The first feed stream comprising alpha, beta unsaturated carboxylic acid is introduced into the lower half of the reactive distillation chamber through a feed line. Generally, this feed stream is introduced as a liquid, for example by pumping. In one embodiment, the feed stream consists essentially of alpha, beta unsaturated carboxylic acid. In an alternative embodiment, the first feed stream may also comprise a mono-alcohol having 5 or more carbon atoms, and the corresponding ester thereof with the alpha, beta unsaturated carboxylic acid. This may be the case when the alpha, beta unsaturated carboxylic acid is pre-mixed with a mono-alcohol having 5 or more carbon atoms, and subjected to a pre-reaction step prior to being fed to the reactive distillation column. However, generally more than 50 % by weight of the alpha, beta unsaturated carboxylic acid which enters the reactive distillation chamber is introduced with the first feed stream.

The second feed stream comprising mono-alcohol is introduced into upper half of the reactive distillation chamber. In one embodiment, the feed stream consists essentially of mono-alcohol having 5 or more carbon atoms. In an alternative embodiment, the second feed stream may also comprise alpha, beta unsaturated carboxylic acid, and the corresponding ester thereof. This may be the case when the mono-alcohol having 5 or more carbon atoms is pre-mixed with an alpha, beta unsaturated carboxylic acid, and subjected to a pre-reaction step prior to being fed to the reactive distillation column. However, generally more than 50 % by weight of the mono-alcohol having 5 or more carbon atoms which enters the reactive distillation chamber is introduced with the first feed stream.

Introducing the first and the second feed stream into the reactive distillation chamber causes a countercurrent flow of acid and alcohol in the reactive distillation chamber, which is beneficial for the reaction rate and conversion. It is preferred that at least one, more preferably both of the feed streams are preheated prior to being fed to the reaction chamber. This increases vaporization and reaction rate in the reactive distillation chamber. In one embodiment, the components are fed as saturated or nearly saturated liquids. By saturated liquid is meant that the feed streams are at such temperature and pressure that any decrease in pressure without a change in temperature causes boiling. However, it is not required that the temperature of the feed streams is adjusted to the exact temperature corresponding to the saturated liquid condition. A very beneficial effect is generally achieved if the temperature is in a range, the upper limit of the range being defined by the temperature corresponding to the saturated liquid condition, and the lower limit of the range being 30 °C, or 20 °C, below the upper limit.

Alcohol and carboxylic acid can be fed into the process at such rates that approximately equimolar amounts of hydroxyl groups from the alcohol and carboxylic acid groups from the acid are introduced per time unit. Typically, the feed molar ratio of alcohol to acid is in the range of 0.95 to 1.20, preferably 1.00 to 1.15. However, it is generally preferred to use a small molar excess of mono alcohol. Preferably, excess alcohol introduced into the reactor can leave the system with the product stream at the bottom of the reactive distillation column, or with the distillate, or both. In one embodiment, the process of the invention comprises the additional step of cycling the liquid or gaseous reaction mixture from the reactive distillation chamber through a loop. This can be advantageous if the reaction rate of the esterification reaction is low. Cycling the reaction mixture through a loop will increase the residence time and therefore increase the conversion of reactants.

The process of the invention can be carried out in the presence of an esterification catalyst. In view of the higher reaction rate, it is generally preferred to carry out the process in the presence of a catalyst. Acidic catalysts are generally suitable as esterification catalysts. The catalyst may be a homogeneous catalyst, which is typically soluble in the liquid organic phase.

Examples of homogeneous catalysts include: sulfuric acid, p-toluene sulfonic acid (pTSA), methane sulfonic acid (MSA), or trifluoro methane sulfonic (triflic) acid. The catalyst may also be a heterogeneous catalyst. Examples of heterogeneous catalysts include: ion exchange resins such as the Amberlyst and Nafion series, sulfated zirconia or titania, mixed metal oxides, zeolites.

It is also preferred to include one or more usual polymerization inhibitors in the process to prevent unintended polymerization of the alpha, beta unsaturated acid or ester. Examples of suitable inhibitors include hydroquinone, hydroquinone methyl ether, hydroquinone ethyl ether, mono-tert-butyl hydroquinone, 2,5-di-tert-butyl hydroquinone, phenothiazine, piperazine di-acrylamide, catechol, p-tert-butyl catechol, p-benzoquinone, 2,5 p-dimethyl p-benzoquinone, anthraquinone, 2,6 di- tert-butyl hydroxy toluene, 2,6-di t-butyl para cresol, butyl hydroxyl toluene, 3,5-di-t- butyl hydroxyl anisol, 2, 5-di -t-butyl hydroxyl anisol, 4-hydroxy-2,6,6-tetramethyl N- hydroxy piperidine, and mixtures thereof.

The polymerization inhibitors are added in the usual amounts, for example in the range of 200 to 2,000 ppm, and can be included in one of the feed streams. Alternatively or additionally, the inhibitor can be added separately, for example at the top of the reactive distillation chamber. Many known polymerization inhibitors are more efficient in the presence of oxygen. Therefore, it is preferred that the process is carried out in the presence of an oxygen-containing gas, for example air. The pressure in the reactive distillation chamber is below atmospheric pressure. There may be a pressure gradient in the reactive distillation chamber. Generally, when reference is made to pressure values, the values refer to the pressure at the bottom of the reactive distillation chamber. Typically, the pressure is in the range of 0.1 to 0.5 bar.

Preferably, the pressure is kept below a preset pressure level which is the pressure limit of operation. This pressure limit depends on the type of ester produced.

We have surprisingly found that the pressure limit can be correlated with the boiling temperature of the corresponding alcohol at atmospheric pressure.

For acrylates the pressure limit, in bar, is given by equation I:

Piim = exp(3.2133 - 0.034 ^* T _b(R0 H))

wherein Tb(ROH) is the boiling point in °C of the mono-alcohol at atmospheric pressure.

For methacrylates the pressure limit, in bar, is given by equation II:

Piim = exp(3.648 - 0.0271 ^* T _b(R0 H))

wherein Tb(ROH) is the boiling point in °C of the mono-alcohol at atmospheric pressure.

Maintaining the pressure below the pressure pnm has been found to provide esters of alpha, beta unsaturated carboxylic acids with a particularly low content of oligomers and polymers. The process of the invention is further illustrated with reference to Figure 1.

The raw materials are pre-heated in heat exchangers (1 ) and (2), to bring them from ambient conditions to saturated liquid. Methacrylic acid (216 kg/hr, b.p. 154.8 °C) is fed to the lower half (3) of the reactive distillation column (4), while 2- ethyl hexanol (329.5 kg/hr, b.p. 184.1 °C) is fed is to the upper half (5) of the reactive distillation column (4) to allow a countercurrent flow for improved interaction of these reactants. A slight excess of methacrylic acid is fed to the column in order to ensure complete conversion of the 2-ethyl hexanol and thus avoid a significant amount of alcohol in the ethyl-hexyl methacrylate product. The equilibrium reaction can take place everywhere in the reactive distillation column (4), providing that both reactants are present along with an acid catalyst. The reaction rate is enhanced by the presence of a catalyst, para-toluene sulfonic acid (pTSA), 1 % by weight on feed streams. The acid catalyst is fed together with the alcohol stream, at the top of the column. The top product stream is condensed in a condenser (12).

The reactive distillation column (4) has 15 theoretical stages in total and it is operated at 0.15 bar and a reflux ratio of 1.2 kg/kg. Under the described operating conditions, the highest temperature is in the reboiler (6) (145 °C). The top distillate stream consists mainly of water with some entrained 2-ethyl hexanol (49.9 kg/hr, 92.0 %wt water and 8.9 %wt 2-ethyl hexanol, at 53.7 °C). The 2-ethyl hexanol is recovered in a decanter (7) operated at 30 °C, and the organics stream (4.35 kg/hr, 91.0 %wt 2-ethyl hexanol and 9.0 %wt water, 30 °C) is recycled back to the reactive distillation column (4) via a recycling line (8). The aqueous stream (45.6 kg/hr, >99.95 %wt water, 30 °C) from the decanter (7) is removed for storage or reuse (9).

The bottom product stream (10) (500 kg/hr, 99.3 %wt 2-ethyl hexyl methacrylate, 0.7% 2-ethyl hexanol, and traces of methacrylic acid and water) is submitted to a cooler (1 1 ) and stored.